During operation of a gas turbine engine using a multi-stage axial compressor, the turbine rotor is turned at high speed by the turbine so that air is continuously introduced into the compressor, accelerated by the rotating blades and swept rearwards onto an adjacent row of stator vanes.
The stator vanes correct the deflection given to the air by the rotor blades and present the air at the correct angle to the next stage of rotor blades.
It is known to provide variable stator vanes (VSVs).
The manner of operation of a known variable stator vane system is described with reference to FIG. 1 which shows a cut-away side view of part of a compressor section of an aircraft gas turbine engine.
In FIG. 1 there is shown the compressor section 10 of an aircraft gas turbine engine. In the tubular casing 12 of the compressor section are mounted sets of stator vanes 14 circumferentially about the central axis of the compressor section. A corresponding set of rotor vanes 16 is mounted downstream of each set of stator vanes 14. Each stator vane 14 terminates at the casing 12 in a stem 18 rotatable in a bush bearing 20 on the outside of the casing, the end of the stem extending beyond the bush.
Located externally of the casing 12 and adjacent each set of stator vanes 14 are actuator rings 22 (also known as unison rings) extending circumferentially round the casing. With each stator vane 14 in a set, the vane stem 18 is connected to the corresponding actuator ring 22 by means of an actuating lever 24. One end of the actuating lever 24 is clamped to the end of the vane stem 18 by means of a bolt 26 so that there is no relative movement between the stem and the lever. The other end of the lever 24 is connected to the actuator ring 22 by a pin 28 which is rotatable in a bush bearing located in the ring.
The actuator ring 22 is arranged so that it may be rotated in a circumferential direction about the central axis of the compressor section, i.e. in either direction of arrow 9. Consequently, rotation of the actuator ring 22 will, by means of the actuating levers 24, cause rotation of each stator vane 14 about its own axis and thus enable the vanes 14 to assume required angles of incidence to the incoming air.
In order to rotate the actuator ring 22, an actuation arrangement (not shown) is provided. The actuation arrangement typically comprises an actuator which is driven by fuel pressure to move an actuating belt which passes around a crank shaft and connects to the actuator ring. As the actuating belt is moved by the actuator, movement of the actuator ring (in a circumferential direction about the central axis of the compressor section) is effected which results in movement of the actuating levers 24 and, consequently, adjustment of the angle of the stator vanes 14 relative to the central axis.
In order to move the actuator ring 22, actuating levers 24 and stator vanes 14, the actuation load applied by the actuator has to overcome the frictional resistance between components (e.g. between the stems 18/bush bearings 20 and the pins 28 located in bush bearings on the actuating ring 22) resulting from aerodynamic loads on the VSVs.
There is a desire to reduce the actuation load. A high actuation load requires a high fuel pressure which places undesirable demands on the fuel system architecture. Excessively high actuation loads resulting in immovable VSVs have been known to result in numerous aborted take-offs which leads to undesirable service interruption.